Optical recording media, such as magneto-optical recording media or phase change recording media, are portable recording media that allow large amounts of data to be recorded at high density, and the growth of multimedia applications in recent years has been accompanied by rapidly increasing demand for media capable of recording large computer files or video files.
Optical recording media generally consist of a multilayer film including a recording layer formed on a plastic or other transparent, disk-shaped substrate. Information is recorded to or deleted from these optical recording media by irradiating the medium with a laser using a focus servo and guide grooves, or using pre-pits while a tracking servo is employed, and signals are reproduced by using the reflected laser light.
The most common type of magnetic recording medium used to involve so-called light modulation recording, in which erasure was performed by adding a stationary magnetic field, after which recording was performed by adding a stationary magnetic field in the opposite direction, but a magnetic field modulation approach, in which laser light is applied while the magnetic field is modulated according to a recording pattern, has been put in to practical use as a method that allows recording in a single rotation (direct overwrite) and also affords accurate recording even at high recording densities at high speed. Phase change recording media have also been commercialized because direct overwriting is possible with light modulation recording, and reproduction is possible with the same optical system as with a CD or DVD.
The limit to recording density with an optical recording medium is a function of the diffraction limit (≈λ/2NA; where NA is the numeric aperture of the objective lens) determined by the laser wavelength (λ) of the light source. More recently, a system has been proposed in which an NA of 0.8 or higher is obtained by using a pair of objective lenses, and much development has gone into this system. The laser used for recording and reproduction has conventionally irradiated the recording film through the substrate, but the larger the NA, the greater is the astigmatism produced by substrate tilt as the light passes through the substrate, for example, so the substrate must be made thin. In this case, for example, the substrate having thickness of 0.5 mm or less cannot be held during a manufacturing process of a medium, so a method in which information is recorded and reproduced through a protective layer (or a protective sheet) on the thin film is proposed.
Also, as recording density has risen in the field of magnetic recording, MR (magneto-resistive) heads that make use of a magnetic resistance effect have become the most common type of reproduction head, and more recently GMR (giant magneto-resistive) heads that afford higher magnetic field sensitivity have come onto the market. In addition, TMR (tunnel magneto-resistive) heads that give even higher magnetic field sensitivity have been developed. Thus, higher recording density has been achieved with magnetic recording media than with optical recording media as a result of improvements to the medium and the development of practical GMR heads, TMR heads and so forth, but for even higher densities to be attained with magnetic recording media, it is essential that there are improvements such as techniques for increasing the density on a recording film, thermal stability, heat-stability and to disk head interface technology.
A technique for achieving high density recording by the fusion of optical technology with magnetic recording and reproduction technology has also been developed. For instance, a ferrimagnetic recording medium and a thermally-assisted magnetic recording and reproduction system that makes use of a laser beam and this magnetic recording medium have been developed (see Japanese Patent 2,617,025, for example).
With a thermally-assisted magnetic recording and reproduction system such as this, the temperature of the magnetic recording medium is raised by the laser beam during recording, which lowers the coercive force of the recording region, and in this state an external magnetic field is applied with a recording-use magnetic head so that information is recorded in the recording region. During reproduction as well, the temperature of the magnetic recording medium is raised by the laser beam, which increases the strength of residual magnetization in the recording region, and information is reproduced by using a reproduction-use magnetic head (such as a GMR) to detect the magnetic flux from this residual magnetization. The result is the high density magnetic recording and reproduction and information.
Furthermore, with a magneto-optical recording medium, a technique in which the apparent reproduction signal is increased by domain wall movement has been proposed (see Japanese Laid-Open Patent Application H6-290496, for example), but there were problems in terms of the high density recording of fine recording magnetic domains on a recording film.
Furthermore, in the case of magnetic recording, smaller recording domains and higher density have made the thermal stability of the recording magnetic domain a matter of great import, so the stability of the recording magnetic domain, and reliability as an information storage medium must be ensured.
However, when magnetic recording and reproduction with higher density was performed with the conventional magnetic recording media discussed above, a problem of thermal stability in the recording magnetic domain had to be considered.
In order to stabilize recording magnetic domains, it is necessary to increase the magnetic anisotropy of a magnetic recording medium and raise the coercive force at room temperature, but this requires either the use of a magnetic head that applies a large recording magnetic field, or the use of a thermal assist or the like to raise the temperature and recording with a magnetic head.
However, with a magnetic head that applies a large recording magnetic field, the head becomes larger, so it is more difficult to control the amount of lift, to achieve a high transfer rate, and so forth, and in high density recording other problems are encountered as a result of magnetic field leakage to the surroundings.
Also, with a thermally-assisted magnetic recording and reproduction system, the temperature gradient in the recording film and stray magnetic field cause domain wall movement in the recorded magnetic domains, making it difficult to record fine marks.
Particularly, alloys of rare earth metal-transition metal-based materials can raise the coercive force at room temperature. However, the alloys are amorphous, so movement of the magnetic domain wall resulted in instability and disappearance of the magnetic domain of the tiny recording marks.
Furthermore, regardless of the method, the problems are that it is difficult to ensure sufficient long-term reliability for an information storage medium, and stability is poor in high density recording as a result of smaller recording marks.
Also, with using a reproduction head with high reproduction output sensitivity, such as a GMR head or TMR head, there is the problem that an adequate detection signal will not be obtained if the recording mark is too fine. Furthermore, to reduce interference between adjacent marks, the spacing (flying height) between the magnetic head and the recording medium has to be small, and the head-disk gap must be controlled to the order of just a few nanometers or less.
It is an object of the present invention to provide a method for recording to and reproducing from a magnetic recording medium, a magnetic recording medium, and a recording and reproduction device for the same, with which the stability of fine recording marks can be increased and signal characteristics will be excellent, even when recording at high density in magnetic recording and reproduction using thermal assist. This invention addresses this object as well as other objects, which will become apparent to those skilled in the art from this disclosure.